The present disclosure relates to guidance and tracking systems for tracking flexible implements, such as flexible medical instruments. More specifically, the present disclosure relates to an integrated system of markers (active or passive) and fiber Bragg grating (FBG) arrays arranged for interventional and/or surgical procedures and the tracking of flexible medical implements used in these procedures such as needles, catheters and endoscopes.
Surgical guidance enables surgeons to localize the position of rigid surgical instruments relative to the human body without having complete visual access during surgery. Surgical guidance is routinely used in surgeries that involve anatomical locations such as the spine, brain, hip, ear/nose/throat or other organs.
In general, surgical guidance consists of two steps: The first step includes the acquisition of a three dimensional (3D) data set of a relevant anatomical region of the body. This step may involve single or multiple imaging modalities such as computed tomography (CT), magnetic resonance tomography (MRT), positron emission tomography (PET) and ultrasound (US). The 3D data set may be acquired before and/or during the surgical procedure. In the second step, the spatial position of the body and the spatial relation of the surgical instruments to the position of the anatomical region are tracked during the surgery. The spatial position of this anatomical region is then correlated to its 3D data set using specific image registration techniques. After registration, the spatial position of the surgical instruments can be displayed with a 3D representation of the anatomical region for the surgeon.
Typically, optical-based systems are used for tracking spatial positions during the surgery. These systems are based on two cameras that detect the positions of at least three markers attached to the tracked rigid surgical instruments (for example, mounted with LEDs as disclosed in U.S. Pat. No. 5,921,992, or mounted with reflective probes as disclosed in U.S. Pat. No. 6,061,644).
Flexible medical implements are used in a wide variety of medical procedures such as endoscopy, angiography and biopsies to name a few. These medical procedures utilize flexible medical implements such as endoscopes, catheters and needles. These types of implements cannot be accurately tracked using the optical techniques described above due to the deflections they might experience during procedures.
A Fiber Bragg Grating (FBG) is a type of optical sensor, which can be constructed by exposing a photosensitive fiber to a spatially varying distribution of light to induce a periodic index of refraction change within the core of the fiber. When a broadband light source or a tunable laser is coupled into the waveguide, certain wavelengths will be reflected and transmitted based on the periodicity of the grating (Fresnel Reflection), where the reflected wavelength is known as the Bragg wavelength λB.
Applying strain (ϵ) and a change in temperature ΔT to a FBG causes a relative shift of the corresponding Bragg wavelength (ΔλB/λB), which is given by:
                                                                        [                                                      Δλ                    B                                                        λ                    B                                                  ]                            =                            ⁢                                                                    (                                          1                      -                                              p                        e                                                              )                                    ⁢                  ϵ                                +                                                      (                                                                  α                        Λ                                            +                                              α                        n                                                              )                                    ⁢                  Δ                  ⁢                                                                          ⁢                  T                                                                                                        =                            ⁢                                                                    C                    S                                    ⁢                  ϵ                                +                                                      C                    T                                    ⁢                  Δ                  ⁢                                                                          ⁢                  T                                                                                        (        1        )            
Here, pe is the strain optic coefficient, αΛ the thermal expansion coefficient of the optical fiber and αn the thermo-optic coefficient, which can be combined to linear coefficients for the strain CS and the temperature CT.
Similarly the wavelength broadening ΔλBW is given by:ΔλBW=2neffΛ(1−pe)Δϵ  (2)Δϵ=ϵmax−ϵmin  (3)
Δϵ is the strain gradient across the length of the grating, neff is the fiber core index of refraction and Λ is the FBG periodicity.
However, it is much more common, to use the wavelength shift in a FBG to create highly sensitive temperature sensors and strain gauges for a variety of industrial and scientific applications. These measurements can be used to infer a local bending radius of curvature the FBG undergoes, yielding information about the shape of the fiber and/or a device to which it is securely attached.